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Browsing Jeofizik by Author "Diner, Çağrı."
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Item How does anisotropic focal region change the structure of the moment tensor?(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2021., 2021.) Poyraz, Dilay.; Diner, Çağrı.In this study, we show how the invariants of the moment tensors change for different orientations of sources in a vertical transversely isotropic (TI) focal region. The invariants of the moment tensors, namely their norms, traces and eigenvalues, have physical interpretations such as seismic moment, isotropic-component and radiation pattern, respectively. Hence it is important to know how these values change for a given elasticity tensor of the focal region. These invariants strongly depend on the strength of anisotropy which is related with the variation of the eigenvalues of TI elasticity tensor from its closest-isotropic elasticity tensor. We plotted the values of the projection for each 10-gridded orientations of slip and normal vectors around the unit sphere in order to see the distribution properly. Thus, we plotted the projection of source tensor d onto different eigenspaces. In doing so, various materials were used for data of elastic parameters and eigenvalues such as shale, dry-cracks etc. Then the norms of the elasticity tensors of various materials has been calculated and plotted. The maximum and minimum points in the norm plots and the ratios of the maximum and minimum eigenvalues of various materials were found to be similar to each other. The eigenvalues of moment tensor for different anisotropic focal regions have been shown. Thus, the relation between isotropic component amount and γ values were illustrated. Lastly, the strength of anisotropy is evaluated and related with the invariants of the moment tensors.Item Inversion for elasticity tensor of focal region using machine learning algorithms(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2023., 2023) Ünal, Yılmaz.; Diner, Çağrı.The moment tensor is an essential tool in seismology to examine the structure of seismic sources. The deformation at the focal region, using combinations of force couples arranged in a 3 x 3 matrix, is represented by the moment tensor. The moment tensor can be expressed as a linear combination of the eigenvectors of the anisotropic focal region’s elasticity tensor. The eigenvalues of a vertically transversely isotropic (VTI) elasticity tensor from a occurring moment tensor of a focal region can be obtained, and the precision of this determination depends on the degree of anisotropy. Machine learning optimization involves iteratively enhancing a machine learning model’s precision by reducing the error level. Choosing an algorithm that can effectively sample the search space and identify optimal solutions is necessary to optimise a function. Many algorithms are available for function optimization, but it is crucial to set a baseline to determine the practicable solutions for a given problem. This thesis defines a new objective function (misfit function). The function is proposed for obtaining the elastic parameters of an anisotropic focal region, and these parameters are calculated by using machine learning algorithms such as Grid Search, Random Search, Simulated Annealing, and Nelder-Mead.Item Resolution of isotropic percentage in moment tensor inversion of tensile sources(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Öter, Gökçe.; Diner, Çağrı.Moment tensor solutions are commonly used in order to understand earthquake source mechanism. Moment tensor can be decomposed into three components, namely isotropic (ISO), double-couple (DC) and compensated linear vector dipole (CLVD). It is well-known fact that tensile sources generate non-DC components and those earth quakes can be defined as the combination of both tensile and shear motion on a fault. In this thesis, resolution of the isotropic part in the moment tensor are considered for tensile sources. For that reason, synthetic waveforms are created by using full moment tensors with different isotropic percentages and those waveforms are inverted with gCAP method. Afterwards, a range of different isotropic values, with a step of t = 0.1, are forced in the moment tensor inversion process in order to investigate the change in variance reduction as the isotropic percentage deviate from its true value. Inversions of the full waveform are performed in different distances and depths for three moment tensors with different isotropic percentages, namely 2%, 5% and 14%. Inver sions results of those original moment tensors and moment tensors with manipulated isotropic percentages are expressed. Those results are compared to each other in terms of changing isotropic percentages, depth and variance reduction in different stations. The results can be summarized as firstly, inversion is not really sensitive to the isotropic component of the moment tensor because isotropic component has small en ergy compared to the whole waveform. Secondly, earthquakes with relatively high isotropic percentages are less sensitive when inversions are performed for high values of manipulated isotropy. Finally, it is observed that the error in the depth of the earthquake is very sensitive to isotropic percentage.Item Studying the finiteness of large earthquakes with higher order moments of the moment tensor(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Örsvuran, Rıdvan.; Konca, Ali Özgün.; Diner, Çağrı.Finite fault parameters of the large earthquakes can be obtained using kinematic finite fault models which consist of a collection of subfaults. Yet calculating each subfault individually costs time and the fault model needs to be defined a priori for the inversion process. Alternative representation of the source is defining it as a moment tensor density distribution. In this case higher order moments of the distribution can be calculated. Higher order moments can also be used to estimate first order finite-fault parameters and it has the advantage of having less number of unknowns. This characteristic would allow for more rapid fault parameter solutions. The aim of this thesis is to develop a measure on the approximation of finite fault models using higher order moments up to degree two. Synthetic seismograms for both finite fault sources and their higher order moment approximations are generated using infinite homogeneous isotropic medium to identify the similarities between the waveforms. The effects of the receiver azimuths and distances are investigated using using different frequency ranges. It is found that higher order moments can give an approximation of waveform broadness or pulse width rather than the overall shape of the waveforms. Higher order moments also improved the point source approximations at frequencies that are beyond the corner frequency of the event. Fault type, fault strike direction and receiver azimuth influence the higher order moment solutions while distance is an insignificant factor at least for the whole-space medium which is considered in this study.Item Symplectic geometry and Hamiltonian Monte Carlo method(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2022., 2022) Öztürk, Feyza.; Diner, Çağrı.Hamiltonian Monte Carlo (HMC) method is an application of a non- Euclidean geometry to an inverse problem. HMC is a probabilistic sampling method with the basis of Hamiltonian dynamics. One of the main advantages of HMC algorithm is to draw independent samples from the model space with a higher acceptance rate than other Markov Chain Monte Carlo (MCMC) methods. In order to understand how higher acceptance rate is achieved, I have studied HMC in the light of symplectic geometry. Hamiltonian dynamics is defined on the phase space (cotangent bundle), which has a natural symplectic structure, i.e. a differential two-form which is non-degenerate and closed. Hamiltonian function is defined on the phase space, which corresponds to the sum of misfit and the square of the generalized momentum. By using the non-degeneracy property of symplectic form, a vector field can be found in which Hamiltonian function is invariant along the integral curves of the vector field. The invariance of the Hamil tonian function results in high acceptance rate, where we apply accept-reject test to satisfy detailed-balance property. In this thesis, we define some basic concepts and theorems in symplectic geometry, then describe the relation between symplectic geometry and HMC, namely Hamiltonian dynamics. Lastly, we show an implementation for HMC algorithm to a 2D-tomography problem and analyze the tune parameters for application of HMC.Item Three-dimensional resistivity modelling and interpretation of geothermal fields in the Gediz graben by magnetotellurics(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2018., 2018.) Cengiz, Özlem.; Diner, Çağrı.The Gediz Graben hosting several geothermal systems is one of the most promising grabens in terms of temperature and production rate of western Anatolia. In order to provide the most comprehensive understanding about the geothermal systems situated in the graben, specifically about the reservoir types, heat sources and structural controls, 253 MT sites were installed at four different areas of the graben to delineate the electrical resistivity distribution at depth. The wide-band MT data were analyzed by phase tensor analysis, and then the data at 31 selected periods in the range from 0.001 s to 1000 s modeled in three-dimensions (3D). The resulting models reveal three different reservoir types, namely (i) a classical geoelectrical distribution of a high temperature geothermal system, with a prominent highly conductive hydrothermal alteration zone sitting above a more resistive deep reservoir zone, (ii) a deep reservoir zone characterized by fractures within metamorphic rocks in the highly resistive basement and (iii) a shallow reservoir (aquifer) corresponding to the hot springs in the shallow sedimentary layer existing in the Gediz Graben. The heat source of the geothermal systems may be attributed to the heat transfer from the interior of the Earth to the upper crust as a consequence of crustal thinning resulted from the extensional tectonics accompanied by magma intrusions into crust in western Anatolia. The 3D models bring out a well-defined interface between the sedimentary cover and underlying metamorphic basement owing to high resistivity contrast between two layers, characterizing the Gediz detachment fault (GDF). The geothermal fields formed along the southern margin of the graben are spatially coincident with the intersecting zone of two fractures, namely the GDF and high angle normal faults, and the circulation of geothermal fluids in reservoirs are dominantly controlled by these fractured zones and major faults. The crustal scale main graben-bounding fault (MGBF) acts as a conduit through which fluids and heat are transported from deeper parts of the crust to near surface. The meteoric waters percolating deep into the crust through the north dipping normal faults are probably heated up by magmatic intrusions, and some of geothermal waters containing meteoric and magmatic fluids rise up to surface through the permeable faults, in particular through the lower bounding sub-horizontal GDF. Furthermore, 3D resistivity models suggest a thick sedimentary layer (2500-3000 m) in the middle part of the graben basin. The thickness of the sedimentary layer decreases gradually on the northern and southern margins of the graben and becomes much thinner towards the eastern end of the graben. 3D resistivity models also delineate an undulating basement topography under the conductive sedimentary fill of the graben.Item Variations of source parameters due to anisotropic focal region(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Ertuncay, Deniz.; Diner, Çağrı.Seismic sources in anisotropic medium have more complex moment tensor structures compared with the moment tensors of isotropic medium. Shear sources in an isotropic focal medium generate pure double-couple (DC) moment tensors. However in an anisotropic medium, shear sources can generate moment tensors with DC, compensated linear vector dipole (CLVD) and isotropic (ISO) components. The DC, CLVD and ISO percentages of a moment tensor depend on the magnitude and the orientation of the anisotropy. In this study, we choose five fault types namely, left/right strike slip, normal, reverse and dip-slip faults in a medium of different anisotropy classes; transversely isotropic, orthotropic and monoclinic. We rotated the anisotropic elasticity tensors of the medium for every possible orientation and evaluate the moment tensors of each cases. Then moment tensor decomposition is applied and DC, CLVD and ISO components are found. We plot the DC, CLVD and ISO percentages of the moment tensors generated by different fault types and anisotropy classes. By using the DC components, first we obtained fault plane orientation then we calculate the deviation from the original fault mechanism. Effects of anisotropy of the source region on calculated fault parameters are found. Distance from isotropic space of given anisotropic elasticity tensor and P/S wave velocity anisotropy percentages are measured. These percentages are proportional to the distane from isotropy. There is a correlation between distance to isotropy and P wave anisotropy with variation of fault plane parameters and percentages of non-DC components of earthquake source.